Spark plug



June 24, 1969 5, v, BRAY 3,451,110

SPARK PLUG Filed Oct. 5, 1966 FIG. 1

STEWART M BRA) INVENTOR.

'BY ggzwazm ATTORNEYS United States Patent 3,451,110 SPARK PLUG StewartV. Bray, Allen Park, Mich., assignor to Ford Motor Company, Dearborn,Mich., a corporation of Delaware Filed Oct. 3, 1966, Ser. No. 583,580Int. Cl. H01t 13/16; F23g 3/01 US. Cl. 29-25.12 5 Claims ABSTRACT OF THEDISCLOSURE This invention relates to a spark plug and to a method usefulfor the assembly of such a spark plug. This spark plug is conspicuousbecause of the economy with which it may be manufactured and itsexcellent performance.

The instant invention is best explained in terms of the drawings inwhich:

FIGURE 1 is a cross section through a spark plug at one stage ofmanufacture, and

FIGURE 2 is a cross section through the same spark plug after completionof the manufacturing process.

The drawings depict the central portion of a spark plug involving theinvention. The drawings show the essential parts of the spark plug to bea central insulator, a complementary steel shell, a lower metallic sealand an upper seal which may be either metallic or a heat resistingplastic O-ring such as an O-ring fabricated from silicone rubber.

The central insulator and the complementary steel shell are dimensionedso as to effect a press fit between these two parts. The area of thepress fit is indicated by the letters A and B and the double shading.Either the exterior of the central insulator, or the interior of thecomplementary steel shell, or both, are tapered to facilitate theassembly of the press fit. The assembly of the central insulator and thecomplementary steel shell is accomplished by forcing the centralinsulator axially into the interior of the complementary steel shellwith a force of between four and five tons as indicated by the arrow.This type of assembly operation deforms the steel shell and establishesintimate contact between the central insulator and the complementarysteel shell. This intimate contact serves to expedite the flow of heatfrom the central insulator to the steel shell. This deformation of thesteel shell establishes a lasting radial sealing force be- 3,451,110Patented June 24, 1969 tween the steel shell and the central insulator.This assembly force also tends to deform the lower metallic seal and soprevent leaks between the steel shell and the central insulator.

A typical automotive spark plug will have a shell diameter at theinterference fit zone of 0.629 to 0.635 inch and an insulator diameterof 0.635 to 0.644 inch. The length of the interference zone is from0.104 to 0.114 inch. The interference fit is typically 0.015 inch.

After the press fit has been made by axially thrusting the centralinsulator into the complementary steel shell the upper portion of thesteel shell is rolled over the upper sealing ring to complete theassembly.

I claim as my invention:

1. A process for the manufacture of spark plugs comprising providing acentral insulator and a complementary steel shell for the reception ofthe central insulator, the outside diameter of at least a portion of thecentral insulator being significantly greater than the mating interiordiameter of the steel shell, and forcing the central insulator intoplace within the steel shell by means of an axially exerted forcewhereby a radial interference fit is established between the centralinsulator and the steel shell and permanent elastic stress is induced inthe steel shell, said radial interference fit assisting in transferringheat from the insulator to the shell.

2. The process recited in claim 1 in which at least one of the centralinsulator and the steel shell is tapered to facilitate the insertion ofthe central insulator in the steel shell.

3. The process of claim 2 in which the force employed for establishingthe interference fit also serves to establish a tight seal between thelower portion of the central insulator and the steel shell.

4. The process of claim 3 in which the seal is effected by thedeformation of a metallic seal.

5. The process of claim 1 in which an additional upper seal is effectedby crimping a heat resistant rubber O-ring seal in place between theupper end of the steel shell and the central insulator.

References Cited UNITED STATES PATENTS 1,353,785 9/1920 Platt 313-143 X1,450,987 4/1923 Pethoud 313-143 X 2,283,164 5/1942 Brewster 313-1182,309,236 1/1943 Burrell et al. 313-143 X JOHN F. CAMPBELL, PrimaryExaminer. RICHARD BERNARDLAZARUS, Assistant Examiner.

US. Cl. X.R. 313-118, 143

